It is known to monitor and control the “slump,” or fluidity property, of concrete in ready-mix delivery trucks by using sensors to monitor the energy required for rotating the mixing drum (e.g., U.S. Pat. No. 4,008,093) and/or the torque applied to the drum by hydraulic pressure (e.g., U.S. Pat. No. 5,713,633). The higher the amperage or hydraulic pressure needed to turn the drum at a given speed, the stiffer or less flow-able (lower slump) the concrete mixture.
Automated control systems enable transportation of concrete in mixer trucks over longer distances by allowing for adjustment of slump in transit through the addition of water or other liquids. A hydraulic sensor coupled to the hydraulic drive and/or a rotational speed sensor connected to the drum may be used for monitoring purposes. Such sensors can be wirelessly connected to a computer processing unit and a wireless communication system to permit modifications to be made during operation. See e.g., U.S. Ser. No. 10/599,130 (Publication No. 2007/01856A1).
The monitoring of concrete slump involves calibrating the outputs or values obtained from the hydraulic sensor and/or electrical sensor on a mixing truck, and correlating these with slump values obtained using a standard slump cone test. In the standard slump cone test, a 12-inch truncated cone containing fresh concrete is removed to permit the concrete to drop, and the vertical height drop of the concrete is measured (e.g. ASTM C143-05). Concrete having this known slump property is added into a rotatable drum mixer so that a hydraulic or electrical value, obtained as an output from the sensor, can be stored into a memory location and subsequently correlated by a computer processing unit. During the delivery of the concrete to a customer, the concrete stiffens with time as a result of hydration, evaporation, and other factors, and the sensors detect this as increased hydraulic or electrical energy required for turning the mixing drum. The on-board computer processing unit compares the detected energy value obtained from the sensor or sensors and compares this to a value or value range stored in computer-accessible memory. If the sensors and computer processing unit (CPU) detect that the concrete is beginning to stiffen, the CPU can then be triggered to activate metering or pumping devices to inject water or other liquid (e.g., chemical dispersant) into the concrete to restore the slump to the desired value.
Other methods are available for manually measuring workability (which is defined by the American Concrete Institute as the ease and homogeneity with which the concrete can be mixed, placed, consolidated, and finished), but these have not been disclosed or suggested for use in automated monitoring systems.
As an example, for highly flowable concrete mixtures such as self-consolidating concrete, the “slump flow” test is used (e.g. ASTM C 1611-05). In this test, concrete is placed in a standard slump cone, the cone is removed, and the horizontal spread—rather than the vertical drop—is measured. For highly flowable mixtures, measurements of the slump (vertical drop) cannot be used because such measurements would not give meaningful information about the workability of the concrete.
The present inventors believe that a major problem of current slump monitoring information obtained by using automated systems is that such equipment provides only information about slump. An objective of the present invention, therefore, is to include “slump flow” test data in automated control systems and methods, so as to provide enhanced capabilities in monitoring and controlling cementitious materials during mixing and/or delivery operations.
Concrete batches are loaded into a concrete mixer on the basis of a mixture design, mixture proportion, or mixture recipe. These three terms are used interchangeably herein. The mixture design stipulates the amounts of all materials to be used in the concrete batch. Concrete batched according to a certain design is expected to achieve certain workability—such as slump or slump flow—and certain hardened properties—such as strength and durability.
Therefore, when batching a load of concrete in a mixer, it is important to batch the correct amounts of all materials. Frequently, however, variations occur in the quantity of materials batched, especially in the water content. Having the correct design water is critical to ensuring that design workability and hardened properties are achieved consistently. Even a small change in water can significantly affect the workability and hardened properties. If too much water is added to a batch of concrete, the concrete will segregate and the strength and durability of the hardened concrete will be reduced. If too little water is added, the concrete will lack adequate workability for successful placement and may be described as “sticky” or “viscous.”
Water is added to a batch of concrete in multiple ways: metered batch water, aggregate free water, post-batch truck added water, residual drum water, and unintentionally added water. The aggregate free water is difficult to measure and may be highly variable during a production. Even moisture probes placed in an aggregate bin before the aggregate is loaded into the mixer are of limited accuracy. Truck operators can be instructed to empty the drum of all residual water and to avoid any unintentionally added water; however, this is not always done completely, if at all. The metered batch water is typically the most accurate measurement. Consequently, significant variation may occur in the total water content of a concrete batch. The variations in total water content of the concrete mixture cause variations in the workability and hardened properties. In contrast, the addition of additional cement dispersant (chemical admixture) has a relatively small affect on hardened properties in comparison to water. Therefore, a mixture with excess cement dispersant may be too fluid but have adequate strength, whereas a mixture with excess water content may be too fluid and have inadequate (low) strength.
In current slump monitoring equipment, it is known to add water, chemical admixtures, or both to adjust the slump or hydraulic pressure to a pre-determined target. Both water and certain chemical admixtures—such as cement dispersants—are known to increase slump and reduce the hydraulic pressure to rotate a mixing drum. However, if the slump or hydraulic pressure is above or below the pre-determined target, it is not apparent whether this discrepancy is due to incorrect water or chemical admixture content.
The present inventors believe that a major problem of current slump monitoring equipment is the lack of a method for determining whether to add water, chemical admixtures, or both and in what quantities to add such materials so that the correct workability and hardened properties are achieved consistently for a given mixture design. An objective of the present invention, therefore, is to provide such a method.
Additionally, the present inventors believe that the present invention provides a method for determining whether to add water, chemical admixtures, or both to achieve the correct workability and hardened properties of concrete manufactured in a stationary central mixing device as typically used in concrete mixing plants
Hence, a novel method and system for monitoring and adjusting concrete rheological properties in mixing drums and other mixing devices are needed.